In the field of high frequency applications, the signal conductors of a coaxial electric signal cable are enclosed within one or more layers of conductive shielding to prevent leakage of electromagnetic energy either into or out of the cable. Also, it has been found that conductive jackets can reduce problems of electrostatic discharge in electronic systems. Solutions to these problems have been attempted by using more than one layer of shielding, such as braided metal wire or tape, or multiple layers of metal coated polymer tape to provide an effective shielding. Multiple layers of shielding however usually make a cable relatively inflexible. Problems also occur in terminating such multiple shields to ground or in commonly grounding all layers of shielding. Many of the problems are outlined in detail in the background portions of U.S. Pat. Nos. 4,871,883, 4,371,742, and 4,347,487, and those portions of the references are hereby incorporated by reference.
Recently there has been interest in providing cables having conductive jackets, primarily to reduce problems of electrostatic discharge in electronic systems. When a conductive jacket is used with metal coated polymer tape shielding, a problem arises of how to achieve a conductive path from the jacket to the inner shield to eventually contact the drain wires which ground all conductive shielding layers. The metal side of the metal-coated polymer tape must face the inside of the cable so as to make contact with the drain wires to provide a cable having the best electrical performance. The polymer tape layer upon which the metal layer is coated lies between the metal layer and the conductive outer jacket and thus insulates the metal layer from the conductive outer jacket. One possible solution is to metal-coat both sides of the polymer tape. However, this structure dramatically stiffens the cable and makes processing very difficult.
Other solutions which have been tried include laser-etching of the polymer film in certain areas to expose the metal to the jacket, folding back the edge of the metal-coated polymer tape to expose the edge of metal to the jacket, cutting the aluminized polymer from the metal side of the tape in order to smear the edge toward the jacketed side, and applying the shield with less than 100% coverage (typically 150% coverage or 50% overlap is used) to expose the drain wires to the jacket.
None of these proposed solutions provided reliable contact between the jacket and shield while maintaining flexibility or processability.